Currently, as a size of a display increases, a frequency of a driving circuit is constantly increased, and a thin film transistor with a higher carrier mobility is required to serve as a switch of a pixel unit in the display. The conventional thin film transistor (TFT) adopts an amorphous silicon material as an active layer, with the carrier mobility being only 0.5 cm2/V·s. For a large-sized display exceeding 80 inches, its driving frequency reaches 120 Hz, and correspondingly the active layer of the thin film transistor is required to have a carrier mobility of more than 1.0 cm2/V·s. Obviously, the carrier mobility of the amorphous silicon TFT hardly meets the large-sized display. Therefore, people turn their attention to a metal oxide semiconductor active layer with a higher carrier mobility. Among the metal oxide semiconductors capable of serving as the active layer, ZnON has attracted much attention from people due to the advantages of high carrier mobility (greater than 100 cm2/V·s), low raw material cost, simple manufacture process, enabling the TFT to have higher on-state current, greatly improving a response speed of pixels and suitable for large-sized display.
However, since the carrier mobility of the ZnON active layer is more than 200 times than that of the conventional amorphous silicon active layer, overhigh carrier mobility will cause an increase of leakage current of the TFT. Thus, the performance of the TFT is adversely influenced, the reliability of a TFT array substrate is reduced, and further the display quality of the display is degraded.